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Converging Lens and Sun

  • #1
jacksonpeeble
Gold Member
118
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Homework Statement


A camper is trying to start a fire by focusing sunlight onto a piece of paper. The diameter of the sun is 1.39e9 m, and its mean distance from the earth is 1.50e11 m. The camper is using a converging lens whose focal length is 10.8 cm.
(a) What is the area of the sun's image on the paper?
(b) If 0.568 W of sunlight passes through the lens, what is the intensity of the sunlight at the paper?

Homework Equations


(a) A=.25*pi*(height of sun's image)^2 and m=-di/do
(b) I=P/A

The Attempt at a Solution


B should not be an issue if I can find A. I do not know how to find the height of the sun's image.
 

Answers and Replies

  • #2
collinsmark
Homework Helper
Gold Member
2,894
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Try 1/f = 1/do + 1/di

This is the "thin lens" formula. You'll use it a lot when studying lenses. You might want to commit it to memory.
 
  • #3
jacksonpeeble
Gold Member
118
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Try 1/f = 1/do + 1/di

This is the "thin lens" formula. You'll use it a lot when studying lenses. You might want to commit it to memory.
Thanks for the reply! I really appreciate your quick response! :smile: That is one of the main formulas that we use.

I managed to solve the problem by taking the focal length over the distance earth to sun and then multiplying that by the diameter of the sun.
(.108/ 1.5E-11) * 1.39E9= h
then taking pi/4 * h^2 =A

My problem before was that I was using 10.8 instead.
 

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